While these figures may be taken as showing quite conclusively the dietetic standards adopted by mankind, there is no evidence whatever that they represent the real needs or requirements of the body. We may even question whether simple observation of the kinds and amounts of food consumed by different classes of people under different conditions of life have any very important bearing upon this question. They throw light upon dietetic habits, it is true, but such observations give no information as to how far the diets in question serve the real needs of the body. We may find, for example, that under certain given conditions of diet the people in question have the appearance of being well nourished, and that they do their work with apparent ease and comfort; but might not these same results follow with smaller amounts of food? If so, there must of necessity be a certain amount of physiological economy under the more restricted diet, and a consequent ultimate gain to the body through diminished wear and tear of the bodily machinery. Indeed, experimental work and observations scattered through the last few years have suggested the possibility of much lower standards of diet sufficing to meet the real physiological needs of the body. Thus, Hirschfeld,[10] in 1887, found in experimenting on himself (24 years of age and weighing 73 kilos) that it was possible to maintain nitrogen equilibrium on a diet containing only 5 to 7.5 grams of nitrogen per day, or 35 to 45 grams of proteid, for a period of ten to fifteen days. The amount of non-nitrogenous food consumed, however, was fairly large, especially the amount of butter,—frequently 100 grams a day—the average fuel value ranging from 3750 to 3916 large calories daily. In 1888 Hirschfeld,[11] again experimenting on himself, maintained nitrogen equilibrium for several days on 7.5 grams of nitrogen per day, with fats and carbohydrate sufficient to yield a total fuel value of 3462 large calories as the daily average. The chief criticism of Hirschfeld’s experiments is that he failed to obtain in all cases definite analytical data of the food-stuffs employed and failed to determine the nitrogen of the fæces. Still his results are of value as indicating the possibility of maintaining nitrogenous equilibrium for a brief time at least on a low proteid intake.